Hi Bob
While I cannot comment on compact cameras as I never use one for bird
photography, I would like to clarify one small point with regards to the
AF system of Canon cameras. (do seriously consider a DSLR with a good
prime lens though, compact cameras just don't rate for serious bird
photography in my opinion)
All Canon DSLRs will autofocus with all available and selectable AF points
at a maximum aperture of f/5.6. Therefore, if you put a 300mm f/4L IS USM
lens (which I personally own too) and add a 1.4x converter (meaning a loss
of one stop of light) you get a 420mm lens (300 x 1.4 = 420) with an
aperture of f/5.6. This means that any Canon DSLR will autofocus with all
AF points, including the 7D. There are millions (likely) using the 7D
around the world with a myriad of lenses and if it were only to have the
one available AF point (the center one) then the sales would diminish very
quickly.
Although if you were to put a 2x converter on a 300mm f/4L IS ISM lens it
would be a 600mm f/8 lens due to the extra glass (being the 2x converter)
causing a loss of two stops of light. Then you would only get manual
focus.
However, Canon's 1D series use only the center AF point when a lens with a
maximum aperture of f/8 is attached (from this month I believe the 5D
MkIII will also have the AF at f/8 feature enabled).
For example my 500mm f/4L IS USM lens coupled with a Canon EF 2x converter
(making it a 500 x 2 = 1,000mm lens). Because the 2x converter costs a
loss of two stops of light it means that my lens becomes f/8 not f/4. It
sounds complicated, but it is quite simple once this basic concept of
measuring light in stops is grasped.
Best wishes with your bird photography.
--
Akos Lumnitzer
http://www.amatteroflight.com
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